Conventional methods for controlling noise generally involve passive systems, which may include noise absorption or attenuation members, such as fiberglass ceiling panels or thick carpets. Other passive systems include noise baffles, such as sound deflecting highway barriers. In industrial settings, noise due to the operation of machines can be annoying to neighboring residences. Excessive noise can also potentially cause damage to the hearing of workers due to over-exposure at the workplace. Efforts to curb excessive noise in recent years have included active noise systems which sense the noise from a noise source and create a negative or inverse noise to act as a canceling force.
One known noise reduction technique is to use conventional microphones for sensing the unwanted noise and conventional speakers as actuators for broadcasting the negative or inverse of the noise sensed through the microphones to cancel or block out the noise. Microphone/speaker systems have only limited application because it is usually impossible to place the speakers in the same location as the source of the unwanted noise. Since the source and the speakers cannot be at the same locus, there are blind areas, nodes, and areas of overlap which result in uneven canceling of noise, and even areas where the noise is enhanced rather than reduced.
Microphone/speaker systems are most practical when operated within a controlled environment, such as a small enclosure, a chamber, or a waveguide. One notable success for microphone/speaker systems includes personal noise suppressers such as used by airplane pilots. This application obtains good results because the canceling noise can be delivered to a specific target, the human ear, at close range. Another success for a microphone/speaker system is the active noise canceling of noise in a waveguide such as an air conditioning duct. The controlled structure of the duct enables the canceling noise to have the same effect as if it had originated from the same locus as the source of the unwanted noise.
Microphone/speaker systems are not successful outside confined environments where the unwanted noise is broadcast generally, and where the noise must be reduced over a wide open area. Further, conventional microphones and speakers are relatively fragile, and are not suitable for hostile environments, such as wet, dusty, excessively warm, or vibrating environments. In these environments heavy duty sensors and actuators are required.
Recent developments in noise control have resulted in the use of piezoelectric devices for both sensors and actuators in active noise control systems. In U.S. Pat. No. 5,355,417, Burdisso et al. suggest the use of an array of piezoelectric (PZT) actuators positioned on the inner surface of a jet engine inlet cylinder to provide an interfering or canceling noise field. An additional array of sensors provides feedback information to a controller, which controls the input signals to the PZT actuators. The sensors taught are eddy current sensors which measure the fan speed and generate a signal which is correlated with radiated sound, i.e., an algorithm imputes a sound signal based on the measured fan speed. The error sensors taught are microphones, a preferred version of which is a polyvinyldi-flouride (PVDF) strain-induced film. The range of frequency taught is from 2000 to 4000 Hz.
Although the Burdisso et al. system has been shown to be effective for jet engine inlets, the interfering noise is distributed within a waveguide, i.e., the cylindrical jet engine inlet. It would be advantageous to be able to provide a noise cancellation system which would be effective outside a waveguide.
In U.S. Pat. No. 5,370,340, Pla discloses a jet engine noise suppression system using PZT noise sensors and PZT actuators, and a controller which sends a control signal to the PZT actuators in response to the noise sensed by the PZT sensors. The noise sensors can be set up to sense the air-borne noise or the actual vibration (structure-home excitation) of the jet engine. A tachometer provides input regarding the fan rotation rate to the controller. The PZT actuators are designed to provide good impedance matching with the acoustic field inside the engine shroud. The disclosure is limited, however, to noise cancellation systems in a waveguide.
It would be advantageous to have a system for eliminating or reducing unwanted noise for use in non-waveguide applications. Such a system should be capable of operating in harsh environments.